According to the crystallographic structures, the stainless steels can be categorized into the austenitic type, the martensitic type, and the ferritic type. Stainless steels have superior corrosion resistance and are suitable to be used as structural or decorative parts, such as screws, nuts, shafts, pins, decorative accessories, and casings of watches, mobile phones, electronic products and electric appliances. However, the surface hardness and wear resistance of the traditional stainless steels are usually unable to meet application requirements. For example, 316L stainless steel, a designation of AISI (American Iron and Steel Institute), which contains 15-18 wt % Cr, 12-15 wt % Ni, 2-3 wt % Mo, and the balance of iron and impurities, has a hardness of HRB50-70, and the surface thereof is likely to be damaged by abrasion or collision.
A nitriding method or a carburizing method is usually used to generate nitride or increase the concentration of carbon in the surface of a stainless steel workpiece so as to enhance the surface hardness. The carburizing method, in particular, is extensively used in the industry. Normally, stainless steel is carburized in a carbon-bearing atmosphere at a specified temperature for a long period of time. Thereby, carbon atoms can implant into the surface of a workpiece to form a carburized layer. In a U.S. Pat. No. 7,468,107, a stainless steel workpiece is carburized in a methane-bearing atmosphere at a temperature of 1,900-2,000° F. At such high a temperature (over 980° C.), the chromium in stainless steels is likely to react with carbon in the methane-bearing atmosphere. Thus, the amount of dissolved chromium in the surface of the stainless steel workpiece decreases, and the corrosion resistance of the stainless steel workpiece degrades. Accordingly, the carburizing temperature of 316L stainless steel workpiece is preferred to be below the nose in the continuous cooling transformation (CCT) diagram shown in FIG. 1.
The surface of the stainless steel workpiece usually has a passive layer hindering implanation of carbon atoms and impairing formation of a carburized layer when carburization is undertaken at a temperature below the nose temperature. Therefore, the passive layer should be removed before low-temperature carburization. U.S. Pat. Nos. 5,792,282, 5,556,483, and 5,593,510 disclosed a carburization method for austenitic stainless steel, wherein stainless steel is placed in a fluorine- or fluoride-bearing atmosphere at a temperature of 250-450° C. for tens of minutes to convert the passive layer into a fluorinated layer. Next, stainless steel is carburized at a temperature of 400-500° C. Carbon atoms can more easily pass through the fluorinated layer than the passive layer containing chromium oxide. Thus, the carburized depth may reach about 20 μm, and the hardness may reach about HV800, in the abovementioned prior arts.
A U.S. Pat. No. 6,547,888 disclosed modified low-temperature case hardening processes, wherein stainless steel is placed in an N2 atmosphere containing 20 vol % HCl at a temperature of 550° F. for 60 minutes to activate the passive layer. Then, the stainless steel is carburized at a temperature of 880-980° F. In addition, U.S. Pat. Nos. 6,461,448 and 6,093,303 disclosed other low temperature case hardening processes, wherein stainless steel is placed in a fusion salt bath containing a mixture of a cyanide salt, a metal halide salt, and calcium carbide, wherein the cyanide salt and the metal halide salt are used to activate the passive layer of stainless steels, and wherein calcium carbide is the carbon source for carburization.
However, the abovementioned carburizing methods are only suitable for the austenitic stainless steels, which have high solid solubility of carbon. The non-austenitic stainless steels, such as the martensitic stainless steel and the ferritic stainless steel, have low solid solubility of carbon atoms and are hard to be carburized.
At present, the industry usually adopts a solid-solution strengthening method or an age-hardening method to enhance the mechanical properties of the non-austenitic stainless steels. However, the abovementioned two methods cannot effectively improve the surface mechanical properties (such as hardness and wear resistance) of the non-austenitic stainless steels. Thus, the non-austenitic stainless steels are hard to apply to the workpieces demanding high wear resistance or high surface hardness.